1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave resonator, a surface acoustic wave filter, a sharing device, or other suitable device, and more particularly, to a surface acoustic wave device which uses a Shear Horizontal wave (xe2x80x9cSH wavexe2x80x9d).
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been widely used as band-pass filters for use in mobile communication equipment. One such conventional surface acoustic wave devices include a surface acoustic wave resonator having an IDT (interdigital transducer) composed of interdigital electrodes having electrode fingers interdigitated with each other, the IDT being disposed on a piezoelectric substrate, and a surface acoustic wave filter using the surface acoustic wave resonator.
In such a surface acoustic wave device, a technique is known in which a leaky surface acoustic wave having a large attenuation which propagates in a Y-X LiTaO3 substrate with Euler angles (0xc2x0, xe2x88x9290xc2x0, 0xc2x0) as a piezoelectric substrate is converted to a Love wave type surface acoustic wave having no propagation loss by providing an IDT having a predetermined thickness and made of a metal having a large mass load such as Au, Ta, W, or other suitable metal.
FIG. 11 is a graph showing the variation of the electromechanical coupling coefficient k with the film thickness H/xcex of Au electrodes (electrode film thickness/wavelength of excited surface acoustic wave), when the Au electrodes are provided on an LiTaO3 substrate of Y cut X propagation type, that is, having Euler angles of (0xc2x0, xe2x88x9290xc2x0, 0xc2x0).
As shown in FIG. 11, a leaky surface acoustic wave is produced when the film thickness H/xcex of the Au electrodes 0.03 or less. In the range of H/xcex of at least 0.004, a Love wave is produced. FIG. 12 is a characteristic graph showing the propagation loss (attenuation constant) of the leaky surface acoustic wave under the same conditions as those of FIG. 11. The solid line represents the propagation loss when the electrodes are in the electrical short-circuiting state, and the dotted line represents the propagation loss when the electrodes are in the open-circuiting state. As shown in FIG. 12, in the electrical short-circuiting state, the propagation loss is zero in the range of H/xcex of about at least 0.033, and in the electrical open-circuiting state, the propagation loss is zero in the range of H/xcex of about at least 0.044. Accordingly, to use an SH type surface acoustic wave having no propagation loss, the thickness H/xcex of the Au electrodes in the electrical short-circuiting state is required to be at least 0.033, depending on the metalization ratio of the IDT. Further, for material such as Ta, W or other suitable material having a lower density than Au, the thickness H/xcex must be more than 0.033.
However, as the thickness of IDT increases, the production accuracy decreases. Accordingly, a sufficiently large thickness cannot be achieved. Unless the film thickness is adequately large, for example, the thickness H/xcex is at least 0.033 for the Au electrodes, a propagation loss of zero cannot be achieved.
On the other hand, the film thickness H/xcex (electrode thickness/excited SH wavelength) at which the electrode fingers of an IDT can be made with general accuracy is up to 0.05. When the propagation loss is desired to be zero, the film thickness H/xcex is required to be at least 0.033. Thus, the range of the film thickness where the electrode fingers of IDT can be formed with high accuracy is very narrow.
Further, if an IDT is formed of an electrode material having a slightly lower density than Au, such as Ta or W, the thickness of the electrodes must be further increased as compared with that of the Au electrodes. Thus, the propagation loss cannot be reduced to zero in the range of film thickness in which the film can be accurately formed.
Regarding materials such as Au having a considerably higher density as compared with electrodes materials generally used in the IDTs of surface acoustic wave devices such as Al, the frequencies differ with even slight variations in film thickness, electrode finger width, and electrode finger pitch of the IDTs. Thus, after the IDTs are formed, the frequencies are conditioned by trimming the IDTs. However, when an IDT is formed from Au so as to have H/xcex of about 0.034, as an example, but the frequency is less than a desired value, such a frequency conditioning is carried out, causing the film thickness H/xcex to be less than 0.033. That is, the propagation loss cannot be set at zero.
To overcome the above-described problems, preferred embodiments of the present invention provide a surface acoustic wave device in which the IDT are produced with high accuracy, the propagation losses in the IDT and the piezoelectric substrate are approximately zero, and the conditioning range for frequency trimming is substantially wider than the surface acoustic wave devices of the prior art.
According to one preferred embodiment of the present invention, a surface acoustic wave device includes a LiTaO3 substrate and an interdigital transducer provided on the LiTaO3 substrate. The interdigital transducer includes at least one of Au, Ag, Ta, Mo, Cu, Ni, Cr, Zn, and W, and the interdigital transducer has a normalized film thickness H/xcex of about 0.05 or less so as to excite a shear horizontal wave.
If the interdigital transducer includes Au as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-146xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.001 to about 0.05.
If the interdigital transducer includes Ag as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-146xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.002 to about 0.05.
If the interdigital transducer includes Ta as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-140xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.002 to about 0.05.
If the interdigital transducer includes Mo as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-134xc2x0, 0xc2x0xc2x150xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.005 to about 0.05.
If the interdigital transducer includes Cu as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-137xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.003 to about 0.05.
If the interdigital transducer includes Ni as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-133xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.006 to about 0.05.
If the interdigital transducer includes Cr as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-147xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.003 to about 0.05.
If the interdigital transducer includes Zn as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-137xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.003 to about 0.05.
If the interdigital transducer includes W as a major component, the substrate preferably has Euler angles of approximately (0xc2x0, 125xc2x0-138xc2x0, 0xc2x0xc2x15xc2x0), and the standardized film thickness H/xcex is preferably within the range of about 0.002 to about 0.05.
The above-explained surface acoustic wave device is suitable for use in a communication device.
According to preferred embodiments of the present invention, on a LiTaO3 substrate having adequate Euler angles, an IDT is formed from an electrode material having a large specific gravity such as Au, Ag, Ta, Mo, Cu, Ni, Cr, Zn, Pt, W, or other suitable material, at an adequate film thickness, whereby an SH wave having a low propagation loss is excited. Thus, the leaky surface acoustic wave component is significantly reduced. A surface acoustic wave device having a very low propagation loss is produced.
Further, the propagation loss becomes substantially zero even where the film thickness is extremely small. Accordingly, even where the film thickness is altered by trimming IDT to control the frequency, the propagation loss is prevented from deteriorating, in contrast to the conventional surface acoustic wave devices. Thus, the conditioning range of the frequency trimming is much wider than the conventional surface acoustic wave devices.
Other features, characteristics, elements and advantages of the present invention will become apparent from the following description of preferred embodiments thereof with reference to the attached drawings.